1. Field of the Invention
The present invention relates to modems in a computer system, and more specifically to an analog front end of a x Digital Subscriber Line (xDSL) modem.
2. Description of the Related Art
Improvements in computer hardware price/performance ratios during the last decade have caused major shifts in both home and office computing environments. Advances in semiconductor technology have enabled the appearance of personal computers with a computing power equivalent to supercomputers available a decade ago. Further, the advent of high performance audio and graphics subsystems have turned the personal computer into a multimedia device capable of delivering multimedia entertainment to the computer user.
Multimedia is a relatively new capability for computer systems. Multimedia refers to the delivery of information that combines different content formats. For example, these different formats may include a motion video, audio, still images, graphics, animation and text. Historically, magnetic media, such as floppy disk, was the medium of choice for providing multimedia content. Due to the limited storage capabilities of floppy disks, multimedia on floppy disks were limited to small programs such as unsophisticated games, with little graphics details and low quality sound.
Newer multimedia mediums such as compact disk-read only memory (CD-ROM) provide greater storage capabilities. In addition to games, CD-ROMs in the form of digital video disks (DVDs) contain full-length movies. While DVDs continue to exist, other means for providing multimedia to computer systems have become available and newer methods are being developed.
Another multimedia medium called the Internet, and more specifically the World Wide Web (WWW) has gained rapid acceptance among computer users. The WWW is a system of documents and multimedia files that can be retrieved and viewed by any person who has access to the Internet.
The WWW is simply a series a communications and protocols representative of information and documents that could be linked to other documents and stored on computers throughout the Internet. The users of the Internet can access documents or pages via program called a browser. Although earlier browsers were text-only, today's browsers offer windows-based icons, pull-down menus, bit-map graphics and telephone links to display hypertext documents. Furthermore, the web standard presents a system independent graphical user interface for users.
Due to its ability to tie together many disparate sources in its unified, easy to access interface, the WWW is rapidly gaining acceptance as an information delivery media. The accessibility of audio and image files such as video over the WWW is also enabling the Internet to become a central depository suitable for entertainment as well.
Historically, users accessed the Internet via a modem which coupled the computer to an Internet Service Provider (ISP) over a standard plain old telephone service (POTS) line. Originally designed for voice communications, the POTS lines include twisted copper wire pairs, amplifiers and filters. The copper wire pairs were strung from a telephone central office (CO) to various customer premises.
The CO is considered to be a first level switching facility. The COs are then connected to a higher level switching facility and these higher level switching facilities are connected to even higher level switching facilities. The connection of all of these switching facilities form a public switched telephone network (PSTN).
Besides the Internet, other multimedia sources are available to the computer user, such as video on demand, and live broadcasts. Video on demand typically refers to the ability to retrieve various video entertainment programming at the touch of a button. For example, such programming may include movies. Video on demand provides the user the convenience of watching a program at home at their own leisure. Live broadcasts are similar to video on demand, except the programs are live, e.g., a sporting event.
Historically, video on demand and live broadcast services, were provided to customers over the air or by video cables. Transmission over the air or via cable was needed because of the high band width requirements for such services. Typically, for instance, to obtain video on demand services, a customer would have to install a satellite dish for reception. Although satellite dish sizes have decreased over the years (0.3 meters), some customers may not be able to install the satellite dish due to city ordinances or neighborhood covenants. In addition, weather conditions such as snow and rain may affect satellite dish reception. Furthermore, satellite dish users require a clear line of sight between the satellite dish and satellite. Structures, such as trees or other buildings, may affect signal reception. Although not typically affected by weather conditions or line of sight requirements, video on demand via cable TV may not be available to the customer due to the high cost of running a cable to their premise, especially for customers living in rural areas.
An emerging standard known as x digital subscriber line (xDSL) is a modem technology that transforms ordinary POTS lines into high speed digital lines for ultra-fast data access. xDSL also enables access to the Internet, reception of video on demand, video catalogs, and live broadcasts, as well as exciting new interactive multimedia applications such as multi-player gaming.
The xDSL modem has, so far as is known, include an analog front end (AFE) for interfacing the modem to the POTS lines. The AFEs are traditionally powered by .+-.12 volt power supplies. The AFEs typically used .+-.12 volts for efficient differential POTS line driving. For xDSL modems designed on a peripheral component interconnect (PCI) card, operating on .+-.12 volts may not be desirable. For instance, although .+-.12, 5 and 3.3 volts are normally made available to the components on the PCI card, in certain computer modes, such as sleeping or low power modes, .+-.12 volts may not be available.
It is often desirable to have the modem operate even when the computer is in the sleeping mode or low power modes. In these modes, it might be desirable to have a fully operable modem to maintain a communications link with the other modem, else the communications link may drop out or fail. In addition, when the computer is operating in these modes, the modem may be able to receive data, such as electronic mail.
Furthermore, POTS line drivers typically require regulated voltages. Some voltages are not well regulated for PCI cards, e.g., .+-.12 volts might not be as well regulated as the 5 and 3.3 volt supplies. It might be possible to regulate the .+-.12 voltages on the PCI card, however, this would increase cost, and be subject to space constraints from already limited board real estate. In addition, .+-.12 voltages on the PCI card typically require additional power planes, which may result in additional board layers and subsequent introduction of additional noise.